1. Field of the Invention
The present invention relates to a fixing temperature controller for thermally fixing a developer image transferred on paper from a photosensitive body in, e.g., a laser printer.
2. Description of the Related Art
FIG. 1 shows an arrangement of a conventional fixing temperature controller.
This temperature controller has an 8-bit register 32 for storing reference data set beforehand by, e.g., a microprocessor. This reference data is used to hold the surface temperature of a heat roller 15 at a predetermined temperature required for fixing. The surface temperature of the heat roller is detected by a temperature sensor and supplied as a voltage signal corresponding to the detected temperature to a sample-and-hold circuit 33. The circuit 33 holds the analog voltage signal from the temperature sensor circuit 31 and supplies the signal to an A/D (analog/digital) converter 34. The A/D converter 34 converts the voltage signal into a digital value. A digital comparator 35 compares the reference data stored in the register 32 with the detected temperature data supplied from the A/D converter 34. If reference data&gt;detected temperature data, the comparator 35 supplies a HIGH-level signal to a noise reduction circuit 36. If reference data .ltoreq.detected temperature data, the comparator 35 supplies a LOW-level signal to the noise reduction circuit 36.
As shown in FIG. 2, the noise reduction circuit 36 has n series-connected D flip-flops 37.sub.1, 37.sub.2, 37.sub.3, . . . , 37.sub.n which operate in synchronism with clocks from a clock generator 38. An output signal from the comparator 35 is input to the first flip-flop 37.sub.1, and output signals from the flip-flops 37.sub.1 to 37.sub.n are supplied in parallel to a majority decision circuit 39. The circuit 39 detects which of the HIGH or LOW level is larger in number in the n input signals and generates an output signal corresponding to the logic level larger in number. This output signal is supplied as an output signal from the noise reduction circuit 36 to a heater driver 30.
In the fixing temperature controller having the above arrangement, if the detected temperature data from the A/D converter 34 changes as indicated by (A) in FIG. 3, the comparator 35 generates an output signal which goes to HIGH level when reference data&gt;detected temperature data and goes to LOW level when reference data.ltoreq.detected temperature data, as indicated by (B) in FIG. 3. In correspondence with this change in output signal from the comparator 35, the noise reduction circuit 36 generates an output signal as indicated by (C) in FIG. 3. The heater driver 30 supplies power to a heater lamp 14 of a fixing unit when the output signal from the noise reduction circuit 36 is at HIGH level and cuts off the power supply to the heater lamp when it is at LOW level.
In the above noise reduction circuit 36, however, since the majority decision circuit 39 uses the result of operations of the flip-flips 37.sub.1 to 37.sub.n, a comparatively long delay is generated from a timing at which reaching of the surface temperature of the heat roller 15 to a predetermined temperature is detected to a timing at which the power supply to the heater lamp 14 is stopped. As a result, the surface temperature of the heat roller 15 is considerably overshot. Therefore, the fixing temperature cannot be controlled with high precision.